Parallax

:This article is about parallax, the perspective shift. For the DC comic book character see Parallax (comics).

Parallax and measurement instruments

If an optical instrument — telescope, microscope, theodolite — is imprecisely focused, the cross-hairs will appear to move with respect to the object focused on if one moves one's head horizontally in front of the eyepiece. This is why it is important, especially when performing measurements, to carefully focus in order to 'eliminate the parallax', and to check by moving one's head.

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Also in non-optical measurements, e.g., the thickness of a ruler can create parallax in fine measurements. One is always cautioned in science classes to "avoid parallax." By this it is meant that one should always take measurements with one's eye on a line directly perpendicular to the ruler, so that the thickness of the ruler does not create error in positioning for fine measurements. A similar error can occur when reading the position of a pointer against a scale in an instrument such as a galvanometer. To help the user to avoid this problem, the scale is sometimes printed above a narrow strip of mirror, and the user positions his eye so that the pointer obscures its own reflection. This guarantees that the user's line of sight is perpendicular to the mirror and therefore to the scale.

Related Topics:
Galvanometer - Mirror - Eye

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In photography, one also talks about the parallax of a camera viewfinder: for nearby objects, a viewfinder mounted on top of the

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camera will show something different from what the lens 'sees', and people's heads may be cut off. The problem does not exist for the

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single lens reflex camera, where the viewfinder looks (with the aid of a movable mirror) through the same lens as is used for taking the photograph.

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